Semi-trailer testing tips
BPW provides advice on how to present semi-trailers for annual test in a manner that will achieve the best results
One of the major tasks when preparing a semi-trailer for the annual MOT test is making sure that the running gear is in good condition.
It is a good idea prior to the MOT to check the foundation brakes for correct operation. In the case of disc brakes, this entails removing the road wheels and pads.
The calliper saddle should be checked to see that it slides freely on the guide pins with no sticking or tight spots. With the tappets wound back, a pad can be used to check that it will slide easily on its location cheeks and does not stick in any one position.
Check that the rubber boots protecting the guide pins and tappets are not damaged. Replace the pads and adjust in accordance with BPW’s Maintenance Instructions (bit.ly/3zOyuxN). Check that there are no air leaks.
In the case of drum brakes, the hub, wheel and drum should be removed and the dust and debris cleaned out of the drum and off the brake shoes and mounting brackets. Check that the brake shoes are secure on the mountings and that the brake shoe pull-off spring is effective.
Grease the camshaft bearings and lubricate the brake cylinder clevis pin. Ensure that the brake cylinder pull-off springs are in place and effective. Replace the hub, wheel and drum and adjust the brakes in accordance with BPW’s Maintenance Instructions (link above). Check for air leaks.
In both cases (drum and disc brake), check that the friction material is sufficient to last until the next inspection or service. If drums or discs are in poor condition, then replacement or skimming (if enough material is available) should be considered.
If drums are being replaced, then this is not an issue as BPW guarantees their new drums and brake shoes will give 80 per cent contact area when replaced at the same time. The same applies to new pads and discs.
Presenting for test
It is best, wherever possible, to present the trailer fully loaded as this gives the truest test of actual brake performance. However, in some cases, circumstances dictate that the vehicle be tested in the unladen or part-laden state.
In order for a semi-trailer to pass the brake performance part of the MOT, the service brake has to achieve a minimum of 45 per cent of the bogie design weight. The parking brake has to achieve a minimum of 16 per cent of the plated gross trailer weight. The procedure of testing is different for single-axle, tandem-axle and tri-axle trailers.
Tri-axle trailers
These can be presented in any one of three ways for testing: laden, part-laden and empty.
* Laden: The trailer is considered to be laden if the combined axle weights exceed half of the design bogie weight. The preferred situation is where the combined axle weights are more than 65 per cent of the design bogie weight. The service and parking brake must achieve a performance of 45 per cent and 16 per cent respectively. In both cases a pass is awarded if more than half the wheels lock.
* Part-laden: A trailer is considered to be part-laden if it carries any load that is more than its operating equipment or if it is a skeletal trailer that has anything in the container. An empty container is allowed, as in some cases the bare chassis is not sufficiently heavy enough to give adequate adhesion for the brake roller test. This is the worst case in which a trailer can be presented for test, because if it has very little weight on it, it still has to achieve a service and parking performance based on the design weights, and in some cases it will be impossible to achieve a pass. The more than half the wheel locking rule does not apply to a trailer presented in the part-laden condition.
* Unladen: In this case, the trailer can be presented completely unladen and rely on wheel locking to achieve a pass. There are some rules which apply in this case relating to minimum brake force achieved. For the service brake test, the minimum total brake force required depends on the number of wheels locking: for six wheels, 3,000kg; for five wheels, 3,600kg; for four wheels, 4,200kg. If three or less wheels lock during this unladen test, then the total brake performance of all three axles must be 45 per cent of the bogie design weight. For the parking brake test, if all wheels on which the parking brake operates lock, a minimum brake force of 1,500 kg is required. If any wheel fails to lock during the test, then the total brake performance must be 16 per cent of the trailer design weight.
Single and tandem axle trailers
These must be presented for test laden or the load simulator can be used at the testing station. The load simulator is a less than ideal situation as it can be difficult to position through the rear doors of the trailer and to produce enough simulated load to get good readings during the test.
The trailers must achieve 45 per cent brake efficiency for the service brake and 16 per cent for the parking brake. There is no system for allowing wheel locks on these trailers.
It should be noted that there are some things that can go wrong during the process, such as under-utilisation, insufficient load on the trailer, high design weights, poor parking brake performance, early locking and insufficient air pressure.
We will save these subjects for a later article and, as always, the experts at BPW are on hand to help if you do encounter any problems.
